This invention relates to illumination detectors such as charge coupled device (CCD) imaging sensors, or diode sensors, and more particularly to the improvement and stabilization of the quantum efficiency (QE) of such sensors in the near IR, visible UV, XUV and soft x-ray regions of the spectrum.
Charge coupled devices (CCD's) have been chosen for a number of terrestrial and space borne astronomical instruments including the Wide Field/Planetary Camera (WF/PC) used in the Hubble Space Telescope (Lockart, B., SPIE, Vol. 331 (1982)). The eight thinned backside illuminated CCD's used by WF/PC provide high sensitivity in the red, excellent charge transfer efficiency, and low read noise performance. However, the sensors exhibit quantum efficiency hysterisis (QEH) and deliver low sensitivity in the blue and ultraviolet portion of the spectrum (i.e., .lambda.&lt;4500 .ANG.). The present inventor has recognized that the surface of the backside illuminated CCD represented two of three parts of a metal-insulator-semiconductor (MIS) device, namely a semiconductor layer and a thin native oxide, but no metal. In a paper titled Backside Charging of the CCD, SPIE, Vol. 570 (1985) by the present inventor, it was shown that such a structure is inherently unstable because of the uncontrolled nature of signal charge in the surface states at the Si-SiO.sub.2 interface. To circumvent this problem, the technique of backside charging was developed which attempts to control the surface potential by providing a negative static charge at the backside of the CCD. While this technique is very effective in eliminating QEH, and improving the quantum efficiency (QE) for WF/PC, it is a temporary solution and periodic recharging is necessary whenever the sensors are warmed to ambient temperatures. (The sensors on WF/PC will be kept at a constant temperature of -95.degree. after ultraviolet flooding and charging with the sun so the QE problems are avoided by this backside charging technique.)
It is an object of this invention to present an improved technique which permanently solves the QE and QEH problems for CCD imaging sensors. Recognizing that the CCD backside is simlar to a MIS system, but lacking the metal film, the present inventor has contemplated completing the structure by applying a thin metal film to the backside of the CCD, and directly controlling its surface potential by an external voltage source. Such a structure will allow the user to have complete control of the surface potential and provide the necessary accumulation at the backside to collect 100% of the photogenerated signal carriers within the CCD potential wells located at the frontside. This condition is referred to as the QE-pinned condition (J. Janesick, Backside Charging of the CCD, SPIE, Vol. 570 (1985)). Unfortunately, the lack of a perfect insulating layer with the desired optical properties has prevented the realization of this structure; any leakage current through this insulator will be collected as signal charge at the frontside.